Various means have been proposed in the prior art for the measurement of ultrasonic waves by using the Principle of interference. For example, one prior means employs a thin flexible reflective film suspended in water through which acoustic waves pass, and wherein the motion of the film is measured interferometrically. This and similar prior systems have the virtues of high sensitivity, large frequency response, large angular range and large dynamic range, but have several serious drawbacks.
Firstly, since the flexible film is suspended inside a water tank, sound passing through it can (and usually will) strike the walls of the tank and be reflected back through the film. This reflection will interfere with the transmitted wave and will cause an erroneous reading. Secondly, since the film must be quite thin (of the order of 6 microns) and is very delicate, it is sensitive to the large, low-frequency motions of the water caused by normal ambient vibrations and other disturbances. If small, these motions cause no harm. However, if they are large, a laser beam reflected from the film can be sufficiently displaced so as to severely reduce the interference and hence adversely affect the output signal. In extreme cases the reflected light beam may even miss the detection diode completely.
Embodiments of prior systems as above described may be found in prior U.S. patents, and examples of such prior patents are the following, noted during a preliminary search: U.S. Pat. Nos., Stetson et al, 3,434,339; Green, 3,711,823; Langlois, 3,964,052; Mezrich et al, 3,969,578; Erikson, 3,990,296.